Collision of molecular anions of benzenedicarboxylic esters with oxygen in a triple quadrupole mass spectrometer

A series of symmetrical phthalate, isophthalate, and terephthalate ester molecular anions were reacted with oxygen in the collision cell of a triple quadrupole mass spectrometer to produce characteristic M − R]⁻ fragments that can be used to identify these compounds. The M − R]⁻/M^−· intensity ratios decreased for homologous esters in the following order: phthalates > isophthalates terephthalates. Based on the M − R]⁻ ion intensities for different alkyl substituents, on ¹⁸O₂ labeling experiments, and on the reactivity of bis(t-butylcyclohexyl)phthalates, it was concluded that M − R anions are generated through an S_N2 nucleophilic displacement at the alpha carbon of the saturated alkyl substituent. For the phenyl ester, the reaction proceeds through attack at the carbonyl carbon.